Electronic materials such as a semiconductor silicon substrate, a liquid crystal glass substrate, or a photomask quartz substrate require highly cleaned surfaces, and wet washing has been performed by various methods using a high-purity dilute chemical solution obtained by diluting a predetermined chemical solution with ultrapure water. In this case, air bubbles may enter the supplied ultrapure water to cause various troubles. For example, in a lithography process of applying a resist to a thin film laminated on a semiconductor wafer, exposing the resist through a mask formed with a pattern, and performing developing and then etching to form the pattern on the thin film, spin coating of the semiconductor wafer with a resist liquid or a development liquid containing air bubbles causes troubles such as a pattern defect due to uneven treatment.
For this reason, various washing methods for preventing air bubbles from entering a dilute chemical solution are disclosed. Patent Document 1 discloses a method for producing a washing chemical solution in which in order to reduce dissolved oxygen in a dilute hydrofluoric acid aqueous solution used in a semiconductor washing step, the dilute hydrofluoric acid aqueous solution is diluted with pure water with a dissolved oxygen content of 10 ppm or less, and most of gas in contact with the dilute hydrofluoric acid aqueous solution in the washing step is rendered inert at room temperature (a volume ratio of oxygen of 21% or less) to prevent an increase in dissolved oxygen in the dilute hydrofluoric acid. Patent Documents 2 and 3 disclose technologies in which in order to reduce dissolved gaseous species in a chemical solution, deaeration means using a vacuum pump is provided in a chemical solution storage tank or a line for conveying the chemical solution to perform deaeration, thereby preventing dissolution of the dissolved gaseous species in the chemical solution.